Boron neutron capture therapy is based on the nuclear reaction that occurs when a stable isotope, 10B, is irradiated with low energy (0.025 eV) or thermal neutrons to yield helium nuclei (α-particles) and 7Li nuclei.
The therapeutic potential of this reaction was recognized by Locher over 50 years ago (Locher, G. L. et al., Am. J. Roentgenol. Radium Ther., 36, 1-13 (1936)), but it was Sweet (Javid, M. et al., J. Clin. Invest., 31, 603-610 (1952); Sweet, W. H., N. Engl. J. Med., 245, 875-878 (1951); Sweet, W. H. et al., J. Neurosurg., 9, 200-209 (1952)), who first suggested that boron neutron capture therapy (BNCT) might be useful for the treatment of brain tumors.
Shortly thereafter, a clinical trial was initiated at the Brookhaven National Laboratory in cooperation with Sweet and others at the Massachusetts General Hospital utilizing borax as the capture agent (Farr, L. E. et al., Am. J. Roentgenol., 71, 279-291 (1954); Godwin, J. T. et al., Cancer (Phila.), 8, 601-615 (1955)). The objective at that time was to use BNCT as an adjunct to surgery for the treatment of patients with the most highly malignant and therapeutically refractory of all brain tumors, glioblastoma multiforme.
Further trials were carried out in the early 1960s, but these failed to show any evidence of therapeutic efficacy (Farr, L. E. et al., supra; Godwin, J. T. et al., supra; Asbury, A. K. et al., J. Neuropathol. Exp. Neurol., 31, 278-303 (1972)) and were associated with adverse effects in normal tissues (Asbury, A. K. et al., supra). Stimulated by the more encouraging clinical studies of Hatanaka et al. (Hatanaka, H. A., J. Neurol., 209, 81-94 (1975); Hatanaka, H. et al., Boron Neutron Capture Therapy for Tumors, Chap. 25, pp. 349-378. Niigata, Japan: Nishimura Co., Ltd. (1986)) for the treatment of malignant gliomas and those of Mishima et al. (Mishima, Y. et al., Lancet., 2, 388-289 (1989)) for melanoma, there has been renewed national and international interest in BNCT.
The theoretical advantage of BNCT is that it is a two component or binary system, consisting of 10B and thermal neutrons, which when combined together generate high linear energy transfer (LET) radiation capable of selectively destroying tumor cells without significant damage to normal tissues. In order for BNCT to succeed a critical amount of 10B and a sufficient number of thermal neutrons must be delivered to individual tumor cells.
Over the past few years the Department of Energy and the NIH have renewed funding for BNCT-related research, and this has supported a growing number of investigators in many different disciplines. Advances in BNCT in the areas of compound distribution and pharmacokinetics compare favorably with other emerging modalities such as photon activation therapy, photodynamic therapy, and the use of radiolabeled antibodies for cancer treatment in which physiological targeting is used.
There are a number of nuclides that have a high propensity for absorbing low energy or thermal neutrons, and this property, referred to as the neutron capture cross-section (σ), is measured in barns (1 b=10−24 cm2). Of the various nuclides that have high neutron capture cross-sections, 10B is the most attractive for the following reasons: (a) it is nonradioactive and readily available, comprising approximately 20% of naturally occurring boron: (b) the particles emitted by the capture reaction [10B(n,α)7Li] are largely high LET: (c) their path lengths are approximately 1 cell diameter (10-14 μm), theoretically limiting the radiation effect to those tumor cells that have taken up a sufficient amount of 10B and simultaneously sparing normal cells and (d) the extensive chemistry of boron is such that it can be incorporated into a multitude of different chemical structures.
7Li and α-particles are the primary fission product of the neutron capture reaction with 10B. α-Particles are relatively slow and give rise to closely spaced ionizing events that consist of tracks of sharply defined columns. They have a path length of approximately 10 νm, are high LET, and destroy a wide variety of biologically active molecules including DNA, RNA, and proteins. For these reasons there is little, if any, cellular repair from α-particle-induced radiation injury.
Since the 10B(n,α)7Li reaction will produce a significant biological effect only when there is a sufficient fluence of thermal neutrons and a critical amount of 10B localized around, on, or within the cell, the radiation produced can be extremely localized thereby sparing normal tissue components. Thus, selectivity is simultaneously one of the advantages and disadvantages of BNCT, since it requires delivery of boron-10 to tumor cells in greater amounts than normal cells.
Ideally, boron compounds to be used for BNCT should have a high specificity for malignant cells with concomitantly low concentrations in adjacent normal tissues and blood. Since it is desirable to confine the radiation solely to these cells, an intracellular and optimally intranuclear localization of boron would be preferred.
Several boron-containing derivatives of chlorpromazine have been synthesized (Nakagawa, T. et al., Chem. Pharm. Bull. (Tokyo), 24, 778-781 (1976); Alam, F. et al., Sthralenther. Onkol., 165, 121-125 (1989)) and are being evaluated for their in vivo tumor localizing properties. p-Boronophenylalanine is another compound that is being studied as a potential capture agent for the treatment of melanoma. The rationale for its use is the avidity of melanomas for aromatic amino acids and their subsequent incorporation into melanin (Ichihashi, M. et al., J. Invest. Dermatol., 78, 215-218 (1982); Mishima, Y. et al., Neutral Capture Therapy, 230-236, Niigata, Japan: Nishimura Co., Ltd. (1986)).
Tumor localization has been demonstrated following I.V. administration by means of whole body autoradiography (Coderre, J. A. et al., Cancer Res., 48, 6313-6316 (1988)) and in several patients with cutaneous melanoma following perilesional injection (Mishima, Y. et al., Sthralenther. Onkol., 165, 251-254 (1989)). Stimulated by Mishima's experience, a number of other boron-containing amino acids have been synthesized that potentially could be incorporated in larger amounts into proteins of malignant cells (Hall, I. H. et al., J. Pharm. Sci., 68, 685-688 (1979).
Another approach to the selective targeting of boron to melanomas is based on the observation that thiouracil is preferentially incorporated into melanotic melanomas during melanogenesis (Whittacker, J. R., J. Biol. Chem., 246, 6217-6226 (1971)). This observation provided the impetus for the synthesis of several boron-containing thiouracils (Gabel, D., Clinical Aspects of Neutron Capture Therapy, 233-241, New York: Plenum Publishing Co. (1989)), and these currently are being evaluated in animals.
Two other classes of compounds with a propensity for localizing in malignant tumors are the porphyrins and the related phthalocyanines. The biochemical basis by which these compounds achieve elevated concentration in malignant tumors is unknown, but this observation has served as the rationale for the use of hematoporphyrin derivative in the photodynamic therapy of cancer (Dougherty, T. J. et al., Porphyrin Photosensitization, 3-13, New York: Plenum Publishing Corp. (1981)).
The high concentration of these compounds in tumors and their intracellular localization and persistence have stimulated several groups of investigators to synthesize boronated porphyrins (Kahl, S. B. et al., Neutron Capture Therapy, 61-67, Niigata, Japan: Nishimura Co., Lid. (1986)) and phthalocyanines (Alam, F. et al., Strahlenther. Oncol., 165, 121-123 (1989)) as potential capture agents. Boronated porphyrins appear to be 3-4 times more effective per unit dose in cell culture than the monomeric or dimeric form of Na2B12H11SH (Laster, B. H. et al., Strahlenther. Oncol., 165, 203-205 (1989)). Although liver concentrations of these compounds are also high (Kahl, S. B. et al., supra) this would not limit their use as a capture agent for the treatment of brain tumors.
One final category-of low molecular weight boron compounds are boron-containing purines and pyrimidines and their nucleosides. The rationale for their development is that such compounds may be selectively incorporated into rapidly proliferating tumor cells and trapped within the cell following their conversion to the corresponding nucleotide. Alternatively, these bases and their nucleosides may function as analogues of naturally occurring precursors of nucleic acids and become incorporated into nuclear DNA.
Cytoplasmic or preferably a nuclear localization of all of these boron compounds would be advantageous since the heavy particles resulting from the capture reaction would deliver a greater proportion of their energy to intranuclear targets, thereby permitting lower boron concentrations than would have been required if the compounds were located extracellularly (Gabel, D. et al., Radiat. Res., 111, 14-25 (1987); Fairchild, R. G. et al., Int. J. Radiat. Oncol. Biol. Phys., 11, 831-840 (1985)). Schinazi and Prusoff (Schinazi, R. F. et al., Tetrahedron Lett., 50, 4981-4984 (1978)) have synthesized the first boron-containing nucleoside, 5-dihydroloxyboryl-2′-deoxyuridine, an analogue of thyrnidine, and have shown that it was not cytotoxic to African green monkey (Vero) cells at a concentration level of 1600 μM (Laster, B. H. et al., Neutron Capture Therapy, 46-54, Niigata, Japan: Nishimura Co., Ltd. (1986)). In vitro neutron radiation studies of cells grown in the presence of 5-dihydroxy-2′-deoxyuridine produced a biological effect that was equivalent to a concentration of ˜6 μg 10B/g, which, if attainable in vivo, would be sufficient for BNCT.
During the 1960s and early 1970s, interests developed in the potential use of polyclonal antibodies directed against tumor-associated antigens for the delivery of drugs and radioisotopes to tumors (Pressman, D. et al., Cancer Res., 40, 3001-3007 (1957); Ghose, T. et al., Br. Med. J., 1, 90-93 (1967); Ghose, T. et al., Cancer (Phila.), 29, 1398-1400 (1972)). In 1964, Soloway suggested that antibodies might be used for the selective targeting of 10B to tumors (Soloway, A. H., supra). Hawthorne et al. (Hawthorne, M. F. et al., J. Medicinal Chem., 15, 449-452 (1972)) reported on the incorporation of the diazonium salt from 1-(4-aminophenyl)-1,2-dicarbo-closo-dodecaborane into antibodies directed against bovine serum albumin and polyclonal antibodies directed against human and mouse histocompatability antigens (Hawthorne, M. F. et al., supra).
It was claimed from in vitro experiments that these immunoconjugates were capable of delivering enough boron to human and murine lymphocytes to sustain a lethal 10B(n,α)7Li reaction, as evidenced by reduced viability following neutron irradiation. However, the immunoconjugates contained only 0.2% natural boron by weight, which was equal to 6 atoms of 10B/molecule of antibody. In retrospect, it appears that there must have been some other explanation for the reduced cell viability that was observed. Sneath et al. (Sneath, R. L., Jr., J. Medicinal Chem., 17, 796-799 (1974)) showed that water-solubilizing groups had to be incorporated into protein-binding polyhedral boranes if protein solubility in aqueous systems was to be maintained.
Subsequently, a group of polyhedral borane derivatives containing protein-binding functional groups were linked to IgG molecules by means of the carbodiimide reaction without evidence of precipitation (Sneath, R. L. et al., J. Medicinal Chem., 19, 1290-1294 (1976)).
One final category of macromolecular species that are possibly useful for the delivery of 10B is what may be termed “encapsulating complexes,” such as liposomes, microspheres, and low density lipoproteins (Kahl, S. B. et al., Strahlenther. Onkol., 165, 137-139 (1989)). Theoretically, large amounts of 10B could be encapsulated, and if these encapsulating complexes could be targeted to the tumor by linkage to a monoclonal antibody using existing methodology or targeting an endogenously expressed cell surface receptor, they might be powerful delivery systems. Again, there may be preferential localization in the reticuloendothelial system, and methodology would have to be developed to minimize this and maximize tumor uptake.
Cobalamin
For several years after the isolation of vitamin B12 as cyanocobalamin in 1948, it was assumed that cyanocobalamin and possibly hydroxocobalamin, its photolytic breakdown product, occurred in man. Since then it has been recognized that cyanocobalamin is an artifact of the isolation of vitamin B12 and that hydroxocobalamin and the two coenzyme forms, methylcobalamin and adenosylcobalamin, are the naturally occurring forms of the vitamin.
The structure of these various forms is shown in FIG. 1, wherein X is CN, OH, CH3 or adenosyl, respectively. Hereinafter, the term cobalamin will be used to refer to all of the molecule except the X group. The fundamental ring system without cobalt (Co) or side chains is called corrin and the octadehydrocorrin is called corrole. FIG. 1 is adapted from The Merck Index, Merck & Co. (11th ed. 1989), wherein X is above the plane defined by the corrin ring and nucleotide is below the plane of the ring. The corrin ring has attached six amidoalkyl (H2NC(O)Alk) substituents, at the 2, 3, 7, 8, 13, and 18 positions, which can be designated a-e and g, respectively. See D. L. Anton et al., J. Amer. Chem. Soc., 102, 2215 (1980).
Methylcobalamin serves as the cytoplasmic coenzyme for 5N-methyltetrahydrofolate: homocysteine methyl transferase (methionine synthase, EC 2.1.1.13), which catalyzes the formation of methionine from homocysteine. Adenosylcobalamin is the mitochondrial coenzyme for methylmalonyl CoA mutase (EC5.4.99.2) which interconverts methylmalonyl CoA and succinyl CoA.
All forms of vitamin B12 (adenosyl-, cyano-, hydroxo-, or methylcobalamin) must be bound by the transport proteins, intrinsic Factor and Transcobalamin II to be biologically active. Specifically, gastrointestinal absorption of vitamin B12 relies upon the intrinsic factor-vitamin B12 complex being bound by the intrinsic factor receptors in the terminal ileum. Likewise, intravascular transport and subsequent cellular uptake of vitamin B12 throughout the body is dependent upon transcobalamin II and the cell membrane transcobalamin II receptors, respectively. After the transcobalamin II-vitamin B12 complex has been internalized, the transport protein undergoes lysozymal degradation, which releases vitamin B12 into the cytoplasm. All forms of vitamin B12 can then be interconverted into adenosyl-, hydroxo-, or methylcobalamin depending upon cellular demand. See, for example, A. E. Finkler et al., Arch. Biochem. Biophys., 120, 79 (1967); C. Hall et al., J. Cell Physiol., 133, 187 (1987); M. E. Rappazzo et al., J. Clin. Invest., 51, 1915 (1972) and R. Soda et al., Blood, 65, 795 (1985).
Cells undergoing rapid proliferation have been shown to have increased uptake of thymidine and methionine. (See, for example, M. E. van Eijkeren et al., Acta Oncologica, 31, 539 (1992); K. Kobota et al., J. Nucl. Med., 32, 2118 (1991) and K. Higashi et al., J. Nucl. Med., 34, 773 (1993)). Since methylcobalamin is directly involved with methionine synthesis and indirectly involved in the synthesis of thymidylate and DNA, it is not surprising that methylcobalamin as well as Cobalt-57-cyanocobalamin have also been shown to have increased uptake in rapidly dividing tissue (for example, see, B. A. Cooper et al., Nature, 191, 393 (1961); H. Flodh, Acta Radiol. Suppl., 284, 55 (1968); L. Bloomquist et al., Experientia, 25, 294 (1969)). Additionally, up-regulation in the number of transcobalamin II receptors has been demonstrated in several malignant cell lines during their accelerated thymidine incorporation and DNA synthesis (see, J. Lindemans et al., Exp. Cell. Res., 184 449 (1989); T. Amagasaki et al., Blood, 26, 138 (1990) and J. A. Begly et al., J. Cell Physiol., 156, 43 (1993).
Vitamin B12 has several characteristics which potentially make it an attractive in vivo tumor therapeutic agent. Vitamin B12 is water soluble, has no known toxicity, and in excess is excreted by glomerular filtration. In addition, the uptake of vitamin B12 could potentially be manipulated by the administration of nitrous oxide and other pharmacological agents (D. Swanson et al., Pharmaceuticals in Medical Imaging, MacMillan Pub. Co., New York (1990) at pages 621-628).
A process for preparing 125I-vitamin B12 derivatives is described in U.S. Pat. No. 3,981,863 issued to Niswender et al. In this process, vitamin B12 is first subjected to mild hydrolysis to form a mixture of monocarboxylic acids, which Houts, infra, disclosed to contain mostly the (e)-isomer. The mixture is then reacted with a p-(aminoalkyl)phenol to introduce a phenol group into the B12 acids (via reaction with one of the free carboxylic acid groups). The mixed substituent B12 derivatives are then iodinated in the phenol-group substituent. This U.S. patent teaches that the mixed 125I-B12 derivatives so made are useful in the radioimmunoassay of B12, using antibodies raised against the mixture.
U.S. Pat. No. 4,465,775 issued to T. M. Houts reported that the components of the radiolabeled mixture of Niswender et al. did not bind with equal affinity to IF. Houts disclosed that radioiodinated derivatives of the pure monocarboxylic (d)-isomer are useful in assays of B12 in which IF is used. However, although Houts generally discloses that the monocarboxylic (d)-isomer can be labeled with fluorophores or enzymes and used in competitive assays for vitamin B12 in fluids, a continuing need exists for labeled vitamin B12 derivatives suitable for tumor and organ imaging and therapy.
U.S. Pat. No. 5,739,313 issued to Collins and Hogenkamp reported that cobalamin analogs comprising a linking group and a chelating group optionally comprising a detectable radionuclide or a paramagnetic ion localize in tumor cells and are useful for imaging tumors.
Despite previous efforts to identify a neutron capture agent that localizes in tumor cells in high concentration and is useful to treat cancer, there is currently a need for neutron capture agents that are useful to treat tumors.